The proposed work is directed towards a further understanding of the molecular mechanisms regulating gene expression, with particular emphasis on factors determining the specificity of these processes. We are analyzing the complex interactions of phage and bacterial proteins which regulate the choice between lytic and lysogenic growth by bacterial virus lambda in its host E. coli. In particular we will continue our studies concerning analysis of four phage regulatory genes--cro, cII, cIII, and Q--whose gene products control synthesis of other lambda proteins, and a bacterial gene Hfl which greatly influences the decision between lytic and lysogenic growth. Our recent isolation of phage mutants which do not require the cIII regulatory protein for efficient lysogenization strengthens the view that cIII directly stimulates repressor synthesis. From our work and work of others, we propose that cIII protein is an antagonist of Hfl protein, which itself is an antagonist of repressor synthesis. We shall test the hypothesis that cIII and Hfl proteins regulate the level of cII protein activity. In this model cII protein is directly involved in stimulating repressor synthesis. Our studies on the role of the cro gene have revealed that overproduction of some early protein(s), oridinarily turned off by cro, are lethal to viral development. We shall identify the gene(s) responsible for the defective growth and attempt to determine the role of these genes in normal phage growth. To provide some additional information on the mechanism by which late gene expression is controlled and the possible role of host proteins in this process, we have isolated mutants (derived from lysogenic bacteria) in which Q protein is no longer necessary for late gene expression. We are currently determining whether the Q-independent mutations affect phage or bacterial genes. These studies on phage and bacterial protein interactions will provide information concerning the specificity of macromolecular synthesis and hence on the control of phage growth.